1. Field of the Invention
The present invention relates generally to portable electronic units and, more specifically, to a portable electronic unit having transmission-side means for transmitting a signal and receiving-side means for receiving a signal from the transmission-side means, at least one of which is carried and used by a user, and to a portable electronic unit used as a portable biological information measurement unit such as a heart rate meter for measuring biological information such as a heart beat, for example.
2. Description of the Prior Art
Conventionally, portable biological information measurement apparatuses have been developed for measuring human biological information about heart beats, pulses, and the number of steps taken. The portable biological information measurement apparatus typically has transmission-side means for detecting and wirelessly transmitting biological information about a user and receiving-side means for receiving and reporting biological information from the transmission-side means. A user of the portable biological information measurement apparatus carries and uses at least one of the transmission-side means and receiving-side means thereof.
For example, a portable heart rate measurement apparatus has been developed, which is a kind of a portable biological information measurement apparatus. The heart rate meter itself for detecting a heart beat and wirelessly transmitting heart rate information is pressed and attached to the chest of the user with a chest belt. A wristwatch type unit is used, which has a receiving portion and a reporting portion. Heart rate information from the heart rate meter is received by the receiving portion of the wristwatch type unit and reported by the reporting portion thereof.
The receiving portion typically includes an antenna for converting an electric wave signal received from the heart rate meter into voltage, a high-frequency amplification circuit for amplifying a miniature voltage generated at the antenna, a detection circuit for detecting an amplified signal, a low-frequency amplification circuit and a comparator for demodulating heart rate data. The configuration described above allows the user to know heart rate information detected by the heart rate meter using the wristwatch type unit located away from the heart rate meter itself.
If an electric wave is radiated from a miniature dipole antenna, the electric field strength Eθ at a point a distance r away from the antenna is expressed by the following equation:Eθ=j·(60πl/λ)·Ie(−jβr)·(1/r+1/(jβr2)+1/((jβ)2r3))If, for example, a long wave signal with a signal frequency of about 100 kHz is used, in short-range communications within 1 m or less, a change in electric field strength reaches about 30 dB in a range of 10 cm to 100 cm and increases to 60 dB, another 30 dB up in a range of 1 to 10 cm. To allow data receipt at a distance of 1 m, a receiving circuit requires an amplification degree of at least 60 dB. A change in a communication distance between the transmission side and the receiving side from 1 cm to 1 m will result in a change in input voltage of 60 dB or so at the receiving side.
Meanwhile, a large number of battery-powered portable units cannot take a dynamic range because these portable units have a low source voltage of 2 to 3V. For a receiving circuit with a fixed gain, a change in data pulse width will therefore result, which may cause a problem that a digital signal cannot be demodulated accurately during short-range communications.
To solve the problem described above, a method is commonly used in which a gain control amplification circuit is used in a receiving circuit.
In the invention described in Non-Patent Document 1. a high-frequency amplification circuit is composed of a transistor-based gain control amplification circuit. A gain is controlled by controlling a collector current (or emitter current) of the transistor.
This allows the construction of a gain control amplification circuit having a relatively simple configuration. However, such a gain control amplification circuit requires a large collector current (e.g., 0.5 to 2 mA), thus resulting in problematic large power consumption. The use of a transformer in the high-frequency amplification circuit also makes it problematically difficult to reduce the size of such a circuit, for example.
In an automatic gain control unit described in Patent Document 2, the control amplifier is controlled using an automatic gain control section that uses an output signal from a mixer, i. e., a signal from the upstream side of the intermediate-frequency filter. The automatic gain control section includes a particular weighting filter, which is connected to the output side of the mixer to output a control signal for a control amplifier for gain control through a detector. The detector is an average-value or peak-value rectifier that includes a diode and a capacitor connected to the downstream side thereof.
The configuration described above allows gain control. Besides a detection circuit for signals received, however, a separate detection circuit for gain control is required, which problematically makes the configuration of the unit complicated and the unit expensive.
In addition, the high-frequency amplification circuit will require gain control. The use of the same circuit configuration as described in the above-mentioned Non-Patent Document 1 will therefore raise problems as described above, such as larger consumption.
<Non-Patent Document 1> Transistor Technology, published by CQ Publishing Co., Ltd., Vol. 350. page 351. October 1998
<Patent Document 2> JP-T-2005-513903. paragraphs (0011) to (0019), FIG. 1
The present invention contemplates eliminating the problems described above. An object of the invention is to provide a portable electronic unit that consumes less power. Another object of the invention is to provide a portable electronic unit that has a smaller size.